Next-generation mobile phone standards such as IMT-Advanced demand higher communication speeds and greater communication capacities at levels exceeding 100 Mbps. Schemes that have been studied with the aim of realizing such communication speeds include MIMO (Multiple Input Multiple Output) transmission, in which space-division multiplexing is performed using a communication station that has a plurality of antennas.
In MIMO transmission, distinct data is transmitted simultaneously by a plurality of antennas, on the transmission side, and received data is combined on the reception side; as a result, it becomes possible to increase the capacity of transmission lines, and to realize higher communication speeds. Also, radio waves from the transmission side reach the reception side over a plurality of routes, and hence it becomes possible to suppress deterioration of communication quality even if numerous obstacles are in the way.
In cellular systems, in particular, various methods under study aim at increasing communication speed and capacity on or in the vicinity of cell boundaries where reception signal power is low. Relay transmission by way of a relay station is one such method (Non-Patent Document 1). In relay transmission, power of a signal transmitted by a transmission station is amplified, and the amplified-power signal is retransmitted and received by a reception station, so that the power of the reception signal in the reception station and the channel capacity are both increased as a result. Methods of relay transmission include, for instance, Amplify-and-Forward (AF), where the relay station just amplifies the power of received signals, and Decode-and-Forward (DF), where the received signal is demodulated and decoded, and is thereafter re-decoded and re-modulated.
Performing relay transmission is advantageous in that this ordinarily enables communication also outside areas where communication with a transmission station is possible, and in that channel capacity can be increased by virtue of the diversity effect that is achieved, in the reception station side, through the use of signals transmitted by the transmission station to the reception station and of signals transmitted by the relay station to the reception station.
In a multi-cell environment, however, signals amplified by a relay station that is disposed in a given cell may interfere with a reception station in another cell, the SINR (Signal to Interference and Noise Ratio) may drop on account of the increased interference signal power, and channel capacity may drop. These occurrences, which are particularly noticeable in cases where a downlink reception station or an uplink transmission station are present on or in the vicinity of a cell boundary, may constitute a hindrance to expanding a communication area.
Accordingly, various technologies have been proposed that are aimed, in particular, at improving communication quality in cases of interference from other cells on or in the vicinity of cell boundaries. For instance, Patent Document 1 discloses a technology that involves detecting interference from neighboring cells at a relay station disposed in a given cell, notifying information on the detected interference to a terminal that is present on or in the vicinity of the boundary of the same cell, and removing an interference component in that terminal.
Patent Document 1: Japanese Patent Application Publication No. 2009-105842
Non-Patent Document 1: R. U. Nabar, H. Blocskei, and F. W. Kneubuhler, “Fading relay channels: performance limits and space-time signal design”, IEEE Journal on Selected Areas in Communications, vol. 22, No. 6, pp. 1099-1109, 2004